output: moved code to audio_output_update()

Moved code from syncAudioDeviceStates() to audio_output_update().

src/output_control.h

@@ -35,6 +35,13 @@ bool
 audio_output_open(struct audio_output *audioOutput,
 		  const struct audio_format *audioFormat);
 
+/**
+ * Opens or closes the device, depending on the "enabled" flag.
+ */
+void
+audio_output_update(struct audio_output *ao,
+		    const struct audio_format *audio_format);
+
 /**
  * Wakes up the audio output thread.  This is part of a workaround for
  * a deadlock bug, and should be removed as soon as the real cause is